Using Water and Gold Catalysts to Generate Novel Solutions for Current Energy Challenges: Purification of Hydrogen and Selective Oxidation of Methane

Renewable hydrogen (H2) is touted to be a possible replacement for carbon based fuels in an effort to reduce greenhouse gas emissions. The current H2 production route through steam reforming of natural gas followed by water-gas shift treatment, however, indirectly releases CO2 and accounts for 3-5% of our global energy use. Moreover, the CO content in the effluent H2 stream from the water-gas shift reactor must be lowered for H2 applications that are sensitive to CO poisoning, for instance, fuel cells. In this work, supported gold catalysts in conjunction with water were used to purify hydrogen through preferential oxidation (PrOx) of CO with O2 in hydrogen-rich streams. The PrOx approach is a more energy efficient solution than the currently used CO methanation process. While water promotes O2 activation, it was found to poison the active metal-support interface (MSI) sites and kinetically hinder H2 activation at the MSI. Thus, water plays two distinct roles in enhancing the selectivity for CO oxidation during PrOx. In the current work, the role of water in inhibiting the undesired H2 oxidation reaction is probed with density functional theory, Fourier transform infrared (FTIR) spectroscopy and kinetic experiments. Methane is the primary component of shale gas and has attracted renewed attention to its potential direct conversion into value-added products such as methanol. The currently used two-stage process of steam reforming to synthesis gas, followed by methanol production from synthesis gas is only profitable at very large scale considering the high energy and infrastructure requirements. In contrast, single step conversion of methane to methanol is attractive for distributed processing and yields a liquid product directly. Environmentally benign O2 and water mixtures were found to generate active oxygen species at the Au/TiO2 MSI, avoiding the need to use expensive oxidants such as H2O2. In the current work, the possible pathways for methane upgrade are explored on Au/TiO2 using the activated oxygen species generated from O2/H2O mixtures.